The invention relates to a process for determining the changeover point in the production of plastic injection mouldings and die-castings.
When producing injection mouldings and die-castings of plastics, metals or ceramics, plastic material is injected into the cavity of a mould by means of suitable injection arrangements, after which it solidifies by cooling or is cured. The mould is then opened and the moulding is ejected. Injection takes place in two phases. During the filling phase the mould is filled, for which the injection arrangements are usually controlled to a filling program so that the delivery rate versus time meets certain setpoints. As soon as the mould is filled, as a rule the injection arrangements are controlled during a hold pressure phase so that the pressure in the cavity reaches certain setpoints, which may also be time-dependent, according to a hold pressure program.
It is very important that the changeover point, i.e. the point in time at which the changeover from filling program to hold pressure program takes place, is correctly determined and coincides as exactly as possible with the time at which the cavity is just filled completely. If the changeover is premature, there is a danger that the cavity will not be completely filled yet and uncontrolled filling under hold pressure will ensue, resulting in a moulding susceptible to warping under thermal stressing. If the changeover is too late, excessive pressure will be set up inside the cavity, yielding a moulding that is brittle and susceptible to fracture on account of internal stresses.
It is essential therefore that the complete filling of the cavity should be detected before a drastic pressure rise occurs. One process of this kind (DE-A-41 40 392) attempts to solve the problem by determining approximately the second derivative of the pressure as a function of time from measurements of the pressure inside the cavity performed at fixed time intervals, and when this value exceeds a fixed threshold it is assumed that the changeover point has been reached and switchover to the hold pressure program then ensues.
However, this solution has emerged as insufficiently reliable, because considerable pressure fluctuations may occur prior to the complete filling of the mould cavity also, causing the second derivative to exceed the threshold briefly so that premature changeover results. Within the limits of this known art, the only countermeasure is to set a high threshold, though this is attended by the added risk that the complete filling of the mould will be detected too late and the changeover will not take place in time.
Consequently this invention is based on the desire to provide a process of this kind which detects the complete filling of the cavity dependably and in good time, whilst remaining stable in the event of disturbances due to chance pressure fluctuations during the filling phase.
This task is fulfilled by the invention as defined in the claims. The invention provides a process for determining the changeover point that functions surely and reliably, and largely precludes the production of defective mouldings or die-castings due to premature or belated changeover.
The invention is explained in more detail below with reference to illustrations showing a typical embodiment.